This invention pertains to a method and apparatus for beam steering a spatially fixed microwave antenna array.
The two primary means of beam steering microwave antennas in the prior art have been mechanical scanning on the one hand and electrical scanning on the other hand. Both types of beam steering have been used, for example, in connection with weather satellites. As a particular example, a system known as Scanning Microwave Spectrometer (SCAMS) was used aboard the Nimbus 6 satellite. The SCAMS system is a multi-channel instrument and achieves scanning or beam steering by stepping a reflector in front of each antenna. Each reflector rotates through 360.degree. in 16 seconds. The rotation rates are low and because the beam widths are large (7.5.degree.), contiguity of successive scan lines is attained. However, ground resolutions are not much better than about 145 km. at nadir.
Mechanical beam steering of higher resolution antennas is often difficult or incompatible with a particular application for several reasons. First, the spatial resolution of a microwave radiometer is directly proportional to the area of the antenna aperture. Higher spatial resolution infers a larger antenna with a narrower beam, thus a smaller footprint or resolution cell; and the available observation time for contiguous coverage is usually determined by other factors, especially for spaceborne applications. Thus, to gain contiguous coverage of a scene, large high resolution antennas must be scanned at high rates. The mechanical moments and torques involved and their impact on other system constraints often make mechanically scanned antennas impractical.
As another example of a prior art beam steering arrangement, the Nimbus 6 satellite also includes a system known as Electrically Scanning Microwave Radiometer (ESMR). The ESMR system achieves beam steering by electronically scanning the antenna. The antenna consists of a slotted waveguide array fed by an associated array of ferrite phase-shifters which are in turn controlled by a beam steering computer. Ferrite phase-shifters are devices in which the permeability properties are modified by application of a magnetic field. By varying the dc current through a coil surrounding the ferrite cores, which are series elements in the waveguide distribution, the phase of the feed network is shifted to achieve beam steering.
Unfortunately, the large array of ferrite devices is quite heavy and the total power required to drive the system is large. Beam broadening occurs not only because of the reduced aperture effect but also because the ferrite shifting approach is frequency sensitive. Thus the received energy of the lower end of the bandwidth arrives from a different direction than the energy at the upper end of the band. To minimize this effect, the bandwidth must be constrained, which in turn constrains system sensitivity.